In electrical systems transient voltages, e.g. from a lighting strike, switching equipment on after an outage, etc., may be imparted to a load. In some cases, the transient voltages can constitute a potential risk to sensitive electronic equipment. Various circuit configurations have, therefore, been developed to protect sensitive electronic equipment from transient voltages.
In one such circuit configuration, current associated with the transient voltage may be shunted from the load by a protection circuit coupled in parallel with the load. The protection circuit may include a series combination of a switching element and a limiting element. In general, the switching element may exhibit high impedance during normal operation, but may switch on, i.e. close, to shunt current from the load to the limiting element in the event a transient voltage is imparted to the circuit. The limiting element may exhibit a limited voltage drop under high current conditions to thereby limit the input voltage to the load.
Known parallel protection circuits, however, may require high change in voltage with respect to time and may not operate for all significant transient voltage conditions. Also, such circuits may shunt very high currents from the load and may therefore be physically large to accommodate significant heat dissipation.